In this project we propose to employ and refine substantially the "ecological" approach to migration within a study of gross in- and out-as well as net migration in the United States over the period 1950 to 1980. Particular attention will be given (1) to a detailed description of the migration patterns over the study period, and to the relationship between in- and out- and net migration for different units of analysis; (2) to the conceptual and analytical refinement and empirical development of the ecological perspective and (3) to the testing of alternative ecological models with the objective of developing a perspective capable of explaining variations in migration across areas within a single time period, as well as changes in migration within areas over time.